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RADIO NAVIGATION
Radio navigation is defined as navigation using radio waves for
determination of position or a line of position. Speed and
direction may also be derived from positional information. Many
Radio navigation Systems exist today. The most widely used
available systems are: Global Positioning System (GPS)
GPS is a satellite position-fixing system operated by the US
Department of Defence (DoD), which is space-based with a
constellation of 24 satellites. The system is used for general
navigation on land, sea and air. It also has survey and timing
applications. GPS provides worldwide three-dimensional coverage.
GPS provides two services for position determination. The
Precise Positioning Service (PPS) will provide predictable
positioning accuracy of 22m horizontally but is limited to the
US and allied military and federal government use. The Standard
Positioning Service (SPS) is available to civil, commercial and
other users at the highest level of accuracy that is consistent
with the international interest of the USA. The current policy
of the US DoD is to provide the SPS at a horizontal accuracy of
100m with 95% probability. The signal is degraded deliberately
by use of "Selector Availability" (SA). President Clinton before
he went out of office, agreed to desist from this deliberate
degrading. However, the U.S. Government can always introduce it
again. GPS signals are liable to interruption without prior
notice
Back To Top Global Navigation Satellite System (Glonass)
Glonass is a space-based radio positioning system operated by
the former USSR for worldwide use. The system is similar to GPS
and may be used for general navigation by aircraft and ships,
etc. The accuracy is 100m-horizontal, 150m vertical and 15cm/s
velocity (all 95% probability).
Back To Top Decca Navigator System (DNS)
The Decca Navigator System, generally known as Decca, is a
hyperbolic radio navigation system currently available in UK,
Ireland and Baltic area of Europe. The system uses groups of at
least three ground transmitter stations called chains. Each
chain comprises one Master and two or three Slave stations, 80
-110 km from the Master station. The accuracy of Decca ranges
from 50 -800m and decreases as the distance from the baseline
increases. The accuracy is also subject to night and seasonal
effects, which generally reduces the accuracy by a factor of 6
to 8. There is very poor accuracy to the south of Ireland.
Currently 24 separate transmitting masts provide coverage over
the UK and Ireland. Decca stations are being closed down
throughout Europe. The UK stations are scheduled to terminate in
March 2000. The mast at Mizen Head was closed in 2000 and the
Irish Coastguard for radio communications in emergencies now
uses the mast.
Back To Top Loran-C
Loran-C is an all weather, highly accurate and reliable
hyperbolic radio navigation system that covers most of the
Northern Hemisphere. The system uses groups of at least three
ground transmitter stations called chains. Each chain comprises
one Master and two or three Secondary stations, several hundred
kilometres from the Master station. Unlike Decca, Loran-C is
unaffected by night and seasonal effects and coverage remains
the same throughout the year and also by day and night. The
Northwest European Loran-C System (NELS) is part of the European
Union plan for an independent European Radio navigation System.
A predicted accuracy of 463m (0.25 nautical miles) will extend
up to 1000 km off the south and west coasts of Ireland while an
accuracy of greater than 100m is predicted for the Irish Sea,
the Celtic Sea and the seas to the north of Ireland as well as
all Irish coastal waters. Loran-C accuracy of greater than 100m
will also extend over the whole of Ireland for aero and civil
users. The repeatable accuracy of Loran-C is impressive,
allowing a return to a marked position with greater accuracy
time and time again. All Loran-C signals are constantly being
monitored. A code within the signal, which is known as Blink,
will warn users of any abnormality.
Back To Top Radio Beacons
The Commissioners of Irish Lights have been providing radio
beacons since the first one was established at Mizen Head in
1931. The number and characteristics of radio beacons have
changed over the years to its present complement of eight, as
well as three Calibration beacons. A radio beacon transmits a
Morse code signal, unique to its location, e.g. Mizen Head Morse
Code MZ (-- --··) Tuskar Rock Morse Code TR (- ·-·). This signal
is repeated in a set sequence. The signal allows receivers to
take a bearing of the source transmitters to help determine
position. The frequency of all radio beacons operated by the
Commissioners of Irish Lights is in the band 285- 315 kHZ. The
emission type is A1A (simple keyed carrier signal) and the range
of each radio beacon is 50 or 100 nautical miles. The
Calibration radio beacons transmit only on request and are of
lesser range (nominally 5 nautical miles). They are used to
calibrate ships' Direction Finding (DF) equipment. Except when
providing a Differential GPS overlay, ·-· radio beacons are
scheduled to close down in February 1999.
Back To Top Differential GPS
Differential GPS (DGPS) is a system where the ground reference
station is able to analyse the GPS signal and correct the effect
of Selective Availability (SA). It then re-broadcasts this
correction to suitably equipped receivers. This allows a far
better accuracy than the 100m horizontal accuracy of raw GPS.
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Radio beacon DGPS Service
Marine radio beacons are a suitable means of transmitting
differential corrections including integrity messages to
suitable GPS receivers. Many authorities worldwide, including
the General Lighthouse Authorities for UK and Ireland, have
implemented or plan to implement this service. The accuracy is
better than 5m at the present time. Such radio beacons have a
useful range of 100 to 150 nautical miles.
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Eurofix
Eurofix is a method by which differential corrections to GPS,
including integrity messages are transmitted within the Loran-C
signals. It has been agreed in principle to provide Eurofix from
the Northwest European Loran-C System thereby providing complete
coverage of DGPS over Northwest Europe. A combined receiver
would therefore allow a position fix be computed using 3 methods
- Loran-C, GPS and Eurofix DGPS, with each system checking the
others.
Back To Top Radar Target Enhancers
A Radar Target Enhancer (RTE) is designed to respond to
interrogating radar with an amplified signal, which is
transmitted on the same frequency with minimal time delay. The
effect of this is to provide the structure on which it is
mounted with a consistent radar return where otherwise, without
enhancement, it would have become intermittent, difficult or
impossible to detect.
Back To Top Racons
The term Racon is derived from the first and last ·--· syllables
of the words Radar beacon. This is an accurate description of
the use and purpose of Racons. In their basic form they receive
signals from ships' radars, which trigger the Racon to emit a
characteristic signal, which is in turn received by the ship's
radar. This characteristic signal is in the form of a series of
response pulses which will show up on a ship's radar as a Morse
coded trace and allow easy identification of the particular
Racon being interrogated, eg: Kish Bank Morse code T (-);
Codling Lanby Morse code G (--·). Racons can be placed on any
navigational mark (eg: lighthouses, beacons, perches, buoys,
etc). The return on the ship's radar will clearly identify the
mark from surrounding targets and allow the mariner to
accurately measure his range and bearing. The Commissioners of
Irish Lights are presently implementing major improvements in
the provision of Racons on the Irish coast. When completed, some
22 high specification Racons will be in place. The Racons
provided are state of the art and will respond on standard 3cm
ships' radars (X-band 9300 -9500 Mhz) and on 10cm radars (S-band
2900- 3100 Mhz). They are strategically placed to serve through
traffic, the approaches to major ports, and fishing and leisure
interests.
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